AN108711 - NXP Semiconductors

Philips Semiconductors
1.4.2.1 Initial RF Collision Avoidance / Detection
Application Note
Tips and Tricks for PN51x Integration
The RF Collision Avoidance (RFCA) 6 in active mode is based on two techniques:
• RF field sensing
In discrete time frames the Initiator / Target checks if there is already an RF field
- or during communication - if there is a violation due to an invalid CRC.
• Time Jitter
The initiation of certain commands depends on a randomly chosen time offset. In
the NFCIP-1 protocol there are four possible discrete time frames which are
selected on a random basis. This prevents having deadlock situations.
The RFCA is performed as follows:
The Initiator first checks if already an RF field can be detected. If there is no RF field for a
time greater than TIDT , plus a random time frame (n * TRFW), the initiator activates the RF
field. This time frame gives other initiators the chance to detect that another Initiator has
already started the RFCA (RF Collision Avoidance) sequence. Now the initiator can send
an initial ATR_REQ. After the command has been issued, the initiator switches off the RF
field and waits for an ATR_RES from at least one target. The initiation of the ATR_RES
follows the same strategy as for the ATR_REQ. This means that the target also starts at
a random time frame and also checks if there is already a target, which has already
switched on the RF field. If the Target has already detected an RF field (i.e. there is
another Target in the RF field) it won’t respond by an ATR_RES.
Although the RFCA is sufficient in most cases, it sometimes can happen that at least two
initiators or targets are modulating at the same time. In order to detect these conflicts, the
PN51x checks the hardware calculated CRC values from the ATR_REQ and ATR_RES.
If there was an error the selection process has to be repeated.
After the ATR_REQ/ATR_RES pair has been established, collisions shouldn’t occur any
more. Therefore the time-jitter, which is generated by hardware, is switched off. However,
according to NFCIP-1 CRC checks shall be performed throughout the whole
communication.
1.4.3 Bit Representation
The bit representation depends on the mode and on the bit rate. Table 1 shows the
several combinations, which are covered by NFCIP-1. The modulation scheme has to be
set manually, but this functionality is already provided by the BFL.
Table 1: Modulation Schemes
Operating Modes Speed in kBit/s Initiator Target
Passive Mode 106 Miller Manchester
212, 424 Manchester Manchester
Active Mode 106 Miller Miller
212, 424 Manchester Manchester
ISO/IEC 14443-4 106, 212, 424, 848 Miller Manchester
6. The RF collision avoidance has the same purpose as the anti-collision mechanism in the ISO/IEC
14443-3 standard. However, the way how to select the counterpart is different.
© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
Application note Rev. 01.10 — 11. May 2006 9 of 37